Acute promyelocytic leukemia (APL) is characterized by selective expansion of malignant cells blocked at the promyelocytic stage bearing the specific chromosomal translocation t(15;17)(q22;q21) and an effective clinical response to all-trans retinoic acid (ATRA) treatment. In addition to t(15;17), which produces the fusion protein PML-RARalpha, four other variant chromosomal translocations also have been described in APL patients: t(11;17), t(5;17), t(11;17) and del(17), which produce PLZF-RARalpha, NPM-RARalpha, NuMA-RARalpha and STAT5b-RARalpha fusion proteins, respectively. APL disease with PML-RARalpha, NPM-RARalpha, or NuMA-RARalpha fusion proteins is responsive to ATRA, while APL disease with PLZF-RARalpha or STATSb-RARalpha is ATRA-resistant. A considerable amount of evidence has suggested that these RARalpha fusion receptors cause leukemias by interfering with the function of retinoic acid receptors (RARs). Transgenic mouse studies demonstrate that XRARalpha (where X is one of the alternative RARalpha fusion partners: PML, PLZF, NPM, NuMA or STATSb) plays a pivotal role in the pathogenesis of APL. However, how X-RARalpha fusion proteins cause leukemia, and the molecular and cellular basis for the response to ATRA, are not completely known. The fact that the RARalpha gene is involved in each of the APL-translocations suggests that disruption of normal RARalpha function is critical for disease pathogenesis. We and others have demonstrated that each of the X-RARalpha fusion proteins bind RXRalpha, a heterodimerization partner of RARalpha essential for its transcriptional activity, and nuclear receptor (NR) coregulators, such as coactivator (CoA) and corepressor (CoR). Using CFP-tagged constructs and FRAP technique, we demonstrated that two RARalpha-containing fusion proteins, PML-RARalpha and NuMA-RARalpha, have nuclear localization patterns and nuclear mobility distinct from RARalpha that alter the nuclear localization and mobility of RXRalpha. These findings support our hypothesis that alterations in nuclear localization and mobility of APL fusion proteins as well as changes in nuclear localization and mobility of nuclear receptor coregulators by X-RARalpha, compared to wild-type RARalpha, contribute to the pathogenesis of APL. The real-time visualization of transcription dynamics will be explored in three specific aims: Aim I will further characterize differences in intranuclear organization, dynamics and interactions with other coregulators among unliganded or liganded X-RARalpha fusion proteins compared to wild-type RARalpha using confocal fluorescent microscopy and FRAP analysis. Aim II will determine the effects of unliganded or liganded XRARalpha proteins versus wild-type RARalpha protein on chromatin structure, as well as the different interactions between X-RARalpha proteins and chromatin-modifying coregulators at an integrated lac operator array. Aim III will determine the effects of key interactions identified in the lac operator system on real time gene transcription using cells containing a RARE-integrated array that drives an RFP-pts reporter. These experiments will give new insight into the molecular and cellular pathogenesis of APL and its response to ATRA and may prove useful as part of a rapid throughput drug screening assay for new APL treatments. The systems to be established in this proposal will also provide powerful tools and useful information that could be extended to other cancer investigations.